Problem: $ B = \left[\begin{array}{rr}3 & 1 \\ -2 & 4\end{array}\right]$ $ A = \left[\begin{array}{r}-1 \\ 2\end{array}\right]$ Is $ B A$ defined?
Explanation: In order for multiplication of two matrices to be defined, the two inner dimensions must be equal. If the two matrices have dimensions $( m \times  n)$ and $( p \times q)$ , then $ n$ (number of columns in the first matrix) must equal $ p$ (number of rows in the second matrix) for their product to be defined. How many columns does the first matrix, $ B$ , have? How many rows does the second matrix, $ A$ , have? Since $ B$ has the same number of columns (2) as $ A$ has rows (2), $ B A$ is defined.